Polyamic acid: nanoprecipitation and electrophoretic deposition on porous supports

Abstract

Polyamic acid (PAA, a precursor of polyimide) was synthesized from 4,4′-oxydiphthalic anhydride and 4,4′-oxydianiline. PAA, dissolved in dimethylsulfoxide (DMSO), was precipitated into colloidal particles after its injection into acetone. The resulting particle size distribution was found to depend on aging time of PAA solutions, their concentration, and the manner in which the solutions were mixed with acetone. PAA particles of any size down to 10 nm appeared to be achievable by decreasing the acetone/DMSO ratio. Particles in DMSO/acetone suspensions were found to have a significant negative zeta potential. Therefore, there was no need to add organic bases to form PAA anions, in contrast to all previously published studies on the PAA electrodeposition. EPD was performed onto porous stainless-steel or alumina disks, which are suitable supports (reinforcements) for membranes. The slow evaporation of DMSO residue yielded dried polymer layers, comprised of 50–100 nm PAA globules. The outer surface of layers was usually covered with a very thin, continuous PAA skin. Such supported PAA layers—after a simple imidization step via a heat treatment—could be applied as thermally resistant membranes for gas separation.